1. Field of the Invention
This invention relates to apparatus and methods for stabilizing hole boring equipment, and more particularly, but not by way of limitation, to apparatus and methods for drilling by applying weight on a drill bit while maintaining tension in a drill string so that the drill string is stabilized and wellbore deviation is reduced.
2. Setting of the Invention
Conventional or traditional petroleum drilling methods use hole-boring equipment which includes a string of drill pipe at the bottom of which a string of drill collars is connected and at the bottom of which string of drill collars a drill bit is connected. The drill bit diameter is often at least twice the diameter of the drill pipe.
Weight is applied to the drill bit for urging it against the material to be drilled by slacking off on the drill string to place at least part of the string of drill collars in compression. With weight applied to the bit, the drill string is rotated at a speed which rarely exceeds 150 revolutions per minute and more often is less than 100 revolutions per minute.
The string of drill collars, which hangs below the string of drill pipe, can be whatever length is needed to provide sufficient weight capability for the drill bit; however, it is not uncommon for such string of drill collars to be several hundred feet long. Such a length of drill collars in compression is susceptible to lateral deviation which can cause drill string failure or borehole deviation.
Another shortcoming with conventional drill collars is that they typically have a smaller inner diameter (and a larger outer diameter) than the drill pipe to which they are connected. This smaller inner diameter can cause pressure increases in drilling fluid pumped downhole through the drill string.
Another drilling technique of more recent vintage, at least in its application to the drilling of oil or gas wells, utilizes a drill bit which has a diameter barely larger than the diameter of the drill pipe to which the bit is connected. Weight is applied to this bit by placing a significant portion of the drill string in compression. The entire drill string is rotated at high speeds (e.g., 700-1000 revolutions per minute) compared to the more conventional or traditional petroleum drilling methods. This technique is used for coring and has been used in mining-style "slim-hole" coring techniques.
This latter, relatively high-speed drilling technique has been found to have a particularly significant shortcoming in its application in the petroleum industry where drilling frequently occurs in sedimentary rocks which are not inherently sturdy enough to support the substantial length of the drill string which is in compression and which thus tends to exhibit "whipping" (i.e., cyclical lateral deformation leading to fatigue failures) within the borehole. For the borehole to prevent such "whipping", it would need to remain substantially in-gauge throughout its entire length. This is feasible in the typical mining environment having relatively hard igneous and metamorphic rocks. This is not as readily feasible, however, in the aforementioned petroleum environment which largely includes relatively soft sedimentary rocks, many of which are fluid-sensitive and mechanically unstable. The drill string failures which have been noted in the petroleum industry applications of the aforementioned high-speed drilling technique have been linked to substantial borehole enlargement whereby the stabilizing influence of an in-gauge hole on the rotating drill pipe has been lost.
It has been observed that if a nearly-gauge borehole is maintained, such as through the use of shale-stabilizing drilling fluid, failures of drill strings rotated at speeds upward of 700 revolutions per minute can be prevented or reduced, at least where the drill string is made of new, thick-walled drill pipe. Thus, it appears that the combination of borehole gauge maintenance by carefully formulated fluid and of stiffer drill pipe provides an effective "first line of defense" against drill string failures in high-speed drilling operations.
Another way to implement this "first line of defense" is by "nesting" wherein a tubular member lines the borehole to in effect produce an in-gauge wall which supports the drill string. Nesting is used as a casing technique where one size of drill pipe (also known as drill rod) is used to line the borehole, and a smaller size of drill bit and drill rod is passed through the bore and out the bottom of the larger stationary drill rod. Drilling is then resumed with the smaller equipment. Nesting is also used in the event that the larger drill rod becomes irretrievably stuck. Nesting can be employed in a given hole only once or twice before the clearances become so small as to make further drilling infeasible. It has been observed that nesting can reduce drill string vibration and failure and can allow greater rates of penetration.
Although the "first line of defense" may be suitable in some applications, it may become inadequate as the drilling progresses to greater depths. It is possible at increased depths that (1) the drill pipe size must be reduced (thus reducing strength and precluding nesting) because the hole size is reduced for the same reasons that hole sizes become progressively smaller with depth in conventional oilfield operations, and (2) borehole enlargement will occur, such as simply by accruing with time, or from breakdown of the shale-stabilizing influence of the drilling fluid, or the occurrence of cavernous zones, or mechanical borehole erosion, etc. Thus, as the borehole gets deeper, the drill pipe would become unstable whereupon failures could again occur.
Therefore, there is the need for some type of apparatus and method by which a drill string can be stabilized even where support from a borehole is lost. There is also the need for an apparatus and a method of drilling which incorporates such stabilization. That is, there is the need for a "second line of defense" which achieves the beneficial effects of the "first line of defense" (e.g., reduced drill string vibration and failure and increased rates of penetration, and thus improved reliability and performance of the drilling operation), but even in situations where the "first line of defense" may not be practicable.
Such apparatus and method should be applicable to both conventional, relatively low-speed drilling techniques and the coring, relatively high-speed techniques referred to hereinabove.
With respect to the conventional technique wherein smaller inner diameter drill collars are used, it would also be desirable if such apparatus and method could replace such drill collars with a larger inner-diameter structure.
It would also be desirable to provide a stabilizing apparatus and method or drilling apparatus and method which would at least reduce, if not altogether prevent, deviation from a straight drilling path. Although directional drilling receives considerable attention, the majority of oil wells are classified as "straight" holes, i.e., boreholes with a bottom hole target that is essentially beneath the surface location. Deviation control techniques are often utilized to assure that the borehole trajectory does not depart significantly from a vertical attitude. Also, many directional well plans call for significant segments of the well path to be drilled nearly vertically within just a few degrees of tolerance. Again, deviation control methods are often required to meet the objectives of the plan.
Deviation control is not a major problem in areas where the subsurface rocks are relatively uniform and the bedding planes are essentially horizontal. In such cases the drill bit and drill collars are influenced primarily by gravitational forces to advance vertically. In areas where the subsurface rock layers are tilted, as is frequently the case, there is a natural tendency for the drill bit and the drill collars to advance along a non-vertical path that is either perpendicular or parallel to the bedding planes. Deviation can also be caused by the lateral movement of the drill string occurring during "whipping" when a portion of the drill string is in compression. To counteract downhole deviation forces by applying a restorative force to the drill bit which influences the bit to drill vertically, a bottom hole drilling assembly is provided. This has been achieved through the use of "pendulum" assemblies.
The design of a pendulum assembly is based on mechanical principles which dictate that relatively low weight-on-bit levels be applied to the drill bit in order for the assembly to work. The rate of penetration of a drill bit is strongly influenced by the applied weight-on-bit; therefore, there is a considerable trade-off between how fast the drilling progresses (which affects the drilling costs) and deviation control (which affects whether the target is hit). Thus, this existing deviation control technique either provides insufficient inclination control (to obtain faster drilling, for example) or produces costly reductions in the rate of penetration (so that the target will more likely be hit, for example). Furthermore, where such technique of deviation control is used to counteract deviation resulting from lateral movement of a section of the drill string under compression, such technique merely resists the deviation problem rather than treating it at the source. Therefore, there is the further need for a stabilizing apparatus and method and drilling method and apparatus directed to curing this source of deviation, thereby permitting the application of full weight-on-bit levels and corresponding rates of penetration while obtaining desired deviation control.